US4507792A - PCM Encoder conformable to the A-law - Google Patents

PCM Encoder conformable to the A-law Download PDF

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Publication number
US4507792A
US4507792A US06/476,620 US47662083A US4507792A US 4507792 A US4507792 A US 4507792A US 47662083 A US47662083 A US 47662083A US 4507792 A US4507792 A US 4507792A
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output
pcm
signal
flip
encoder
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US06/476,620
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Kazuo Yamakido
Shiro Hagiwara
Fumiaki Fujii
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Hitachi Ltd
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Hitachi Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B14/00—Transmission systems not characterised by the medium used for transmission
    • H04B14/02—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation
    • H04B14/04—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation using pulse code modulation
    • H04B14/046—Systems or methods for reducing noise or bandwidth
    • H04B14/048—Non linear compression or expansion

Definitions

  • the present invention relates to a PCM encoder.
  • PCM encoder which is an apparatus for converting analog signals such as telephone speech into PCM digital signals.
  • the A-law uses a conversion characteristic for encoding as shown in FIG. 1 and a conversion characteristic for decoding as shown in FIG. 2. Accordingly, the total conversion characteristic for encoding and decoding becomes as shown in FIG. 3. That is, it has smaller quantizing steps for smaller absolute values of signal amplitudes and larger quantizing steps for larger absolute values.
  • VF X is a maximum encoding amplitude
  • VF R is a maximum decoding amplitude of FIG. 3 in which analog inputs X n are close to 0 (zero).
  • digital codes shown in the above diagrams are intended to simplify the explanation and are not A-law output character signals (PCM OUTPUT in FIG. 7) as will be described later. As apparent from FIG.
  • Such conversion characteristic intends to minimize the quantization noise of signals obtained by encoding analog signals with continuous level values and decoding the encoded signals.
  • a noise signal having an amplitude level equal to one-half of the quantizing step level of the encoder, namely, an amplitude level of ⁇ Y 1 , is involved in the output of a decoder. This phenomenon can be termed the "amplification possessed by the conversion characteristic for encoding and decoding".
  • the device converts the noise in a signal band into a low frequency component outside the signal band, so as to prevent the noise from affecting a necessary signal.
  • the encoder having the foregoing conversion characteristic for encoding is furnished with the function of detecting the idle channel state, so as to fix the polarity bit of an encoder output PCM signal in the idle channel state or in the absence of any voice signal.
  • FIG. 1 shows a conversion characteristic for A-law encoding
  • FIG. 2 shows a conversion characteristic for A-law decoding
  • FIG. 3 shows a total conversion characteristic for encoding and decoding based on the A-law
  • FIG. 4 is an enlarged diagram of a part of the characteristic shown in FIG. 3;
  • FIG. 5 is a circuit diagram showing the arrangement of an embodiment of a PCM encoder according to the present invention.
  • FIG. 6 is a time chart for explaining the operation of the embodiment.
  • FIG. 7 is a diagram showing character schemes for explaining the operation of the embodiment.
  • FIG. 5 is a circuit diagram of an embodiment of a PCM encoder according to the present invention
  • FIGS. 6 and 7 are a time chart and a character scheme diagram for explaining the arrangement and operation of the embodiment, respectively.
  • This embodiment is an encoder based on the A-law, in which parts other than an idle channel state detector 9 are the same as those of a hitherto-known PCM encoder.
  • An input analog signal applied to an input terminal 1 is fed into a sample-and-hold circuit 3 by a clock 201 (8 kHz) which is supplied from a first clock distributor 2.
  • a voltage (V IN ) held in the sample-and-hold circuit is applied to one input terminal of a voltage comparator 4.
  • the clock distributor 2 provides a pulse 202 for resetting flip-flop circuits 502-508 within a successive approximation register 5.
  • a nonlinear digital-to-analog (D/A) converter 6 which has the conversion characteristic in FIG. 1 stated before, converts a code signal in the successive approximation register 5 into an analog signal and applies the latter to the other input terminal of the comparator 4 as a reference voltage.
  • the pulse signal 202 has been generated, the output of the register 5 becomes a code expressive of the null voltage, so that the output of the converter 6 becomes 0 (zero) volt.
  • the voltage comparator 4 is supplied with a signal by which the polarity information of the sampled and held analog input signal V IN can be determined (for example, "1" for a positive input signal and "0" for a negative input signal).
  • the polarity information signal is latched in a first flip-flop 501 within the successive approximation register 5 by a clock 203 which is supplied at the next point of time.
  • the clock 203 is connected so as to simultaneously set the second flip-flop 502.
  • the output of the D/A converter 6 produces the lowermost analog voltage level of a fourth segment which corresponds to a digital code "11000000", shown in the encoding conversion characteristic of FIG. 1, for the positive input V IN or a digital code "01000000" for the negative input V IN .
  • the output of the voltage comparator 4 provides the result of decision as to whether or not the amplitude of the held analog voltage V IN is a voltage above or below the level of the fourth segment.
  • This output digital signal of the comparator 4 has the logic of exclusive NOR taken with the foregoing polarity information (the output of the flip-flop 501). The result is applied to the data input terminals of the second and succeeding flip-flops 502-508, and is latched in the second flip-flop 502 by the next clock 204.
  • the output digital code of the comparator 4 is a natural binary code as shown in the column COMP OUTPUT in FIG. 7.
  • the output of the comparator 4 and the polarity bit are applied to the data input terminals D of the flip-flop circuits 502-508 through an EXCLUSIVE NOR circuit, while the clock signals CP of the immediately-preceding flip-flop circuits are simultaneously applied to the set terminals S thereof. Accordingly, the input and output codes of the aforementioned successive approximation register 5 become so-called folded binary codes in which the side V IN ⁇ 0 is symmetric to the case of V IN >0 taken as a reference, as indicated in the column SAR OUTPUT in FIG. 7.
  • the digital code composed of 8 bits which represents a step level in one segment corresponding to each level of the held analog voltage V IN , is found by a successive comparison type A/D converter which is constructed of the comparator 4, the successive approximation register 5 including the EXCLUSIVE NOR circuit, and the D/A converter 6.
  • a digital code which is actually provided from the A-law encoder that is, a PCM output code as shown in the column PCM OUTPUT in FIG. 7 is obtained by inverting even bits in the code of the SAR 5 (recommendation by CCITT, Rec. G. 711). In the embodiment of FIG. 5, this is realized by the use of a PCM output register 7.
  • the PCM output register 7 is constructed of a parallel-in serial-out shift register that consists of flip-flops 701, . . . , 708 which are disposed in correspondence with the respective flip-flops 501, . . . , 508 of the register 5, and a logic gate 709 which controls the serial data output of the output register 7.
  • the parallel input terminals P of the flip-flops 701-708 are supplied with the outputs of the corresponding flip-flops 501-508 under the condition that the code "1" or "0" of the output of each even-numbered flip-flop is inverted.
  • the pre-set terminals PS of all the flip-flops 701 708 are connected in common to line 211 of the first clock distributor 2, and the aforementioned converted code from the register 5 is latched in the flip-flops 701-708 by a pulse signal 211 which is sent from the clock distributor 2.
  • a PCM output is usually connected in a wired-OR arrangement directly with the PCM output of another encoder for the purpose of time-division multiplexing. Therefore, the PCM output needs to be held at a high logic level during the time interval during which the PCM signal is not provided as an output.
  • the logic gate 709 is used, and the arrangement is such that when a synchronizing clock 801 for setting the PCM output time is applied, a second clock distributor 8 generates a pulse 802 with a width corresponding to eight clock cycles of a PCM outputting clock externally supplied to a terminal 805, and a clock 803 for outputting the PCM signal 804 within the time interval.
  • a NOR gate 901 provides a high logic level "1" only when all the outputs of the second to eighth flip-flops 502-508 of the SAR 5 are of the low logic level "0".
  • a first AND gate 902 supplies the latch pulse 211 of the PCM output register 7 to the reset terminal R of the counter 903. Therefore, this counter 903 is reset, and its output Q becomes "0". Accordingly, a second AND gate 904 remains enabled and does not impede the setting of the polarity bit of the SAR 5 in accordance with the state of flip-flop 501.
  • the frequency band of voice signals is 300 Hz-3.4 kHz as stipulated also in the CCITT recommendation referred to above, frequency components below 300 Hz should originally be excluded. Accordingly, the object of the present invention can be attained without affecting various characteristics in the talking state when the idle channel state is determined on the basis of a period of time longer than 1/300 second, in other words, when any excluded signal is below 300 Hz.
  • the counter 903 is formed as a clock counter having 8000/300 ⁇ 27 bits or more because the period in the case of encoding with the successive comparison system is 1/8 ms.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Analogue/Digital Conversion (AREA)
  • Compression, Expansion, Code Conversion, And Decoders (AREA)
  • Transmission Systems Not Characterized By The Medium Used For Transmission (AREA)
US06/476,620 1982-03-26 1983-03-18 PCM Encoder conformable to the A-law Expired - Fee Related US4507792A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP57047203A JPS58165441A (ja) 1982-03-26 1982-03-26 Ｐｃｍ信号符号器
JP57-47203 1982-03-26

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US (1) US4507792A (de)
EP (1) EP0090314A3 (de)
JP (1) JPS58165441A (de)
KR (1) KR840004337A (de)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4740993A (en) * 1986-02-25 1988-04-26 Mitel Corporation Digital companding circuit
US5469475A (en) * 1990-06-15 1995-11-21 U.S. Philips Corporation Transmitter comprising an eletronic arrangement for generating a modulated carrier signal
WO2000007178A1 (en) * 1998-07-31 2000-02-10 Conexant Systems, Inc. Method and apparatus for noise elimination through transformation of the output of the speech decoder
US6201830B1 (en) * 1997-06-11 2001-03-13 Texas Instruments Incorporated Low computation idle transmission method for DSL modems
US6219389B1 (en) * 1998-06-30 2001-04-17 Motorola, Inc. Receiver implemented decoding method of selectively processing channel state metrics to minimize power consumption and reduce computational complexity
US6381266B1 (en) * 1998-09-30 2002-04-30 Conexant Systems, Inc. Method and apparatus for identifying the encoding type of a central office codec
US6954451B1 (en) * 2000-11-21 2005-10-11 Ravesim, Inc. Distributed time-multiplexed bus architecture and emulation apparatus

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5933944A (ja) * 1982-08-19 1984-02-24 Sanyo Electric Co Ltd 量子化ノイズ防止回路

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3688221A (en) * 1971-03-02 1972-08-29 Krone Gmbh Two-stage pcm coder with compression characteristic
US3882489A (en) * 1974-05-15 1975-05-06 Chatillon & Sons John Apparatus for producing a digital electrical representation of a peak value of an analog signal
US4151516A (en) * 1975-08-26 1979-04-24 Bell Telephone Laboratories, Incorporated PCM coder with shifting idle channel noise level

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2841221C2 (de) * 1977-09-23 1983-04-07 Telefonbau Und Normalzeit Gmbh, 6000 Frankfurt Schaltungsanordnung zur Unterdrückung des Verstärkens von sehr kleinen Geräusch- und Nebensprechsignalen in PCM-Systemen

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3688221A (en) * 1971-03-02 1972-08-29 Krone Gmbh Two-stage pcm coder with compression characteristic
US3882489A (en) * 1974-05-15 1975-05-06 Chatillon & Sons John Apparatus for producing a digital electrical representation of a peak value of an analog signal
US4151516A (en) * 1975-08-26 1979-04-24 Bell Telephone Laboratories, Incorporated PCM coder with shifting idle channel noise level

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4740993A (en) * 1986-02-25 1988-04-26 Mitel Corporation Digital companding circuit
US5469475A (en) * 1990-06-15 1995-11-21 U.S. Philips Corporation Transmitter comprising an eletronic arrangement for generating a modulated carrier signal
US6201830B1 (en) * 1997-06-11 2001-03-13 Texas Instruments Incorporated Low computation idle transmission method for DSL modems
US6219389B1 (en) * 1998-06-30 2001-04-17 Motorola, Inc. Receiver implemented decoding method of selectively processing channel state metrics to minimize power consumption and reduce computational complexity
WO2000007178A1 (en) * 1998-07-31 2000-02-10 Conexant Systems, Inc. Method and apparatus for noise elimination through transformation of the output of the speech decoder
US6618700B1 (en) 1998-07-31 2003-09-09 Mindspeed Technologies, Inc. Speech coder output transformation method for reducing audible noise
US6381266B1 (en) * 1998-09-30 2002-04-30 Conexant Systems, Inc. Method and apparatus for identifying the encoding type of a central office codec
US6614839B2 (en) * 1998-09-30 2003-09-02 Conexant Systems, Inc. Method and apparatus for identifying the encoding type of a central office codec
US20040013185A1 (en) * 1998-09-30 2004-01-22 Xuming Zhang Method and apparatus for identifying the encoding type of a central office codec
US7173963B2 (en) 1998-09-30 2007-02-06 Silicon Laboratories Inc. Method and apparatus for identifying the encoding type of a central office codec
US6954451B1 (en) * 2000-11-21 2005-10-11 Ravesim, Inc. Distributed time-multiplexed bus architecture and emulation apparatus

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Publication number Publication date
EP0090314A3 (de) 1986-11-20
KR840004337A (ko) 1984-10-10
EP0090314A2 (de) 1983-10-05
JPS58165441A (ja) 1983-09-30

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